The exhaust gas of an internal combustion engine contains NOx, CO, HC, etc. To purify the gas of these substances, the engine exhaust system is provided with a three-way catalyst apparatus. A three-way catalyst apparatus cannot purify the gas of these substances well before it is raised to the catalyst activation temperature, so at the time of engine startup where the catalyst temperature is low, the three-way catalyst apparatus has to be quickly raised to the catalyst activation temperature.
For this reason, it is proposed to make the combustion air-fuel ratio leaner than the stoichiometric air-fuel ratio right after engine startup so as to make the exhaust gas include a sufficient amount of oxygen and to utilize this oxygen to make the HC and CO etc. in the exhaust gas burn in the three-way catalyst apparatus. Further, it has been proposed that if the three-way catalyst apparatus is raised to the catalyst activation temperature, the combustion air-fuel ratio be made the stoichiometric air-fuel ratio, but when the fact that three-way catalyst apparatus is raised to the catalyst activation temperature is judged by the engine cooling water temperature reaching a set temperature, the set temperature be set lower the lower the engine cooling water temperature at the time of start of engine startup and the combustion air-fuel ratio be made lean regardless of the three-way catalyst apparatus as a whole being raised to the catalyst activation temperature and thereby the three-way catalyst apparatus be prevented from being excessively raised in temperature (for example, see Japanese Patent Publication (A) No. 9-151759, Japanese Patent Publication (A) No. 9-222010, Japanese Patent Publication (A) No. 2004-346777, and Japanese Patent Publication (A) No. 2004-324493).
In the above-mentioned background art, until the three-way catalyst apparatus as a whole becomes the catalyst activation temperature, the combustion air-fuel ratio is made lean, so while there is no problem when the cylinder temperature is low in the first half of the temperature rise, if the cylinder temperature becomes relatively high in the second half of the temperature rise, the exhaust gas will include a relatively large amount of NOx. This NOx ends up being released into the atmosphere without being sufficiently removed.
Therefore, an object of the present invention is to provide an air-fuel ratio control device of an internal combustion engine able to reduce the amount of NOx released into the atmosphere when quickly raising the three-way catalyst apparatus to the catalyst activation temperature right after engine startup.